Polyester is a synthetic material widely used to produce filaments for a wide variety of textile applications, including fibers, yarns, and fabrics. Accordingly, one desirable property of polyester, particularly polyester fiber or yarn, is its dyeability; i.e. its ability to absorb and retain color producing dyes. The extent to which a fiber or yarn can accept dye will appropriately determine the colors or shades of colors of the resulting textile products, and thus the product's ultimate value to the consumer and the marketplace. In many circumstances, fibers or yarns that will not accept dyes, or only accept them in moderate amounts, are less valuable than those which can be dyed more successfully.
In dyeing polyester filaments or fibers, dispersed dyes are often used, and tend to be retained more successfully by the filaments when the polyester has a higher crystallinity where higher crystallinity is also accompanied by increased crystalline and amorphous volume. These regions are influenced by manufacturing conditions and thus can be controlled to a certain extent so that the degree of crystallinity will influence the physical properties of the fibers.
There are a number of methods for increasing the crystallinity of a particular fiber or filament, including polyester. Some of these are, however, rather complex or capital intensive. For example, the crystallinity of a polyester filament can be increased by increasing the winding speed at which a particular filament is formed. This is demonstrated, for example, by FIG. 1 in which differential thermal analysis is used to analyze polyester formed at different winding speeds. FIG. 1 demonstrates that the exothermic peak that characterizes the change of amorphous polyester becoming crystalline, although pronounced at winding speeds of 2,000 m/min, decreases significantly at higher winding speeds and disappears completely at about 5,500 m/min. FIG. 1 also illustrates that, at winding speeds between 2,000 and 6,000 meters per minute (m/min), the melting point of polyester (PET) remains essentially constant at about 250.degree. C. In other words, the crystallinity of the spun polyester, and the dyeability of the resulting draw textured fiber, can be increased by increasing the winding speed during production.
An increase in winding speed, however, is not a viable option in many circumstances. For example, increasing the winding speed generally requires an entirely new or significantly enhanced spinning apparatus and a corresponding large capital expenditure. Thus, a method of increasing the crystallinity of polyester, and thus its dyeability, without such capital expenditure in increasing spinning speeds would be desirable.
In addition to dyeability, synthetic fibers such as polyester also desirably should have characteristics that mimic natural fibers in their desirable attributes; for example, breatheability, hand, thermal properties, appearance and the like. Typically, because synthetic fibers are formed as filaments, if they were combined directly into yarns, fabrics, and then garments, the resulting products would have a harsh, shiny, "plastic-looking" appearance that many consumers would find undesirable. Furthermore, their physical properties would more resemble plastic than natural textile products.
Thus, synthetic yarns such as polyester are usually treated or "textured" (or "texturized") to improve their end use characteristics. As used herein and as commonly known in this art, the terms "textured" or "texturized" describe synthetic materials manufactured as continuous filaments that are then crimped or have had random loops imparted, or that have been otherwise modified to create a different surface texture. Common texturizing techniques include air jet texturizing, stuffer box texturizing, gear crimping, edge crimping, and false twisting. In false twisting, yarn is taken from a supply and fed at a controlled draw ratio through a heating unit, and then through a false twist apparatus such as a spindle or friction surface. The twist is set into the yarn by the action of the heater and is subsequently removed at the spindle or other device resulting in a group of filaments that have a tendency to form helical springs. When woven or knitted into fabric, the cover, hand and other aesthetics of the finished fabric more closely resemble the properties of fabrics constructed from natural or other spun yarns.
In recent years, a desirable method of texturizing has been the false twist draw texturizing of partially oriented yarn (POY). In conventional processes for preparing false twisted polyester yarn, the synthetic polyester is melt spun to form a multifilament yarn which is then draw textured to provide a break elongation of about 30%. The drawing phase of the process aligns and arranges the molecular structure of the polyester molecules to obtain desired tensile properties. Simultaneously, the yarn is false twist texturized to provide the desired bulk and other texturizing properties. Typically, the texturizing step incorporates twisting the yarn, heat setting the twist in the yarn at a temperature of above 200.degree. C., and then untwisting the yarn.
Feed yarn, however, is normally produced at a much greater rate than the rate of false twisting. For example, typical draw-texturing processes operate at speeds of 500 to 900 meters per minute, and consume POY yarns at the rate of 300 to 600 meters per minute (accounting for draw ratio). By comparison, feed yarn is produced at rates as high as 4000 m/min. In turn, the speed of feed yarn production is limited by available wind-up speeds. As a solution, the industry has combined drawing with texturizing in an effort to provide a more desirable arrangement. Accordingly, the POY technique partially orients polyester filament during melt spinning, and then both draws and texturizes the partially oriented yarn in a second subsequent step to give the final product.
Partially oriented yarn has gained widespread use in draw texturizing processes. Accordingly, because of the need to dye polyester, and because of the above mentioned capital expense and inconvenience of producing high winding speed polyester to increase the crystallinity of POY, it would be most desirable if a method existed for increasing the crystallinity of partially oriented yarn in conjunction with draw texturizing to produce a draw textured polyester yarn with increased dyeability that can be produced on conventional spinning and draw texturizing equipment without excessive revamping of such equipment.